Molecules labeled with radioactive isotopes have been used as both imaging agents in medical diagnosis as well as therapeutic agents in the treatment of cancer. Both radiolabeled small molecules and radiolabeled peptides and nucleosides have been used to diagnose tumors. In addition to their use as diagnostic tools, radiolabeled nucleotides have been used to treat tumors in mammals by injecting or infusing radiolabeled nucleosides directly to the affected site.
One practical issue associated with the use of radioisotopes is the means by which the radioactive isotope is bound to the delivery molecule. This is important because it is often the case that a molecule with special binding properties will be used to deliver a radioactive isotope to a specific location ill an organism. Hence, it is critical that the functional groups used to bind the radioisotope do not alter the binding specificity of the delivery molecule. Furthermore, the radioisotope should be strongly bound to the delivery molecule because inadvertent release of the radioisotope would unnecessarily subject healthy tissue to radiation.
One common method of labeling molecules with radioactive isotopes for medical use is a stannylation process. See U.S. Pat. No. 5,565,185. Although this process yields isotopically pure products, toxic tin by-products often remain and must be separated before the radiolabeled molecules can be used. In addition, the unstable nature of radiolabeled molecules and their precursors lead to a short shelf-life. Hence, a method for attaching the radioisotope to a wide variety of molecules that avoids toxic side products would be highly desirable.
Radiolabeling of biosequences may also be achieved with activated esters. This method presents a similar problem of chemical purity and isotopic purity. While it is possible to attach a radioactive agent, for example, a benzamide, to a protein or peptide, only a small fraction of the resulting proteins or peptides actually bear the radioactive tag. Separation of the radiolabeled material from non-radiolabeled material is particularly difficult since the protein or peptide is very large and the tag represents only a minor structural modification.
One technique used to simplify the purification of compounds is to attach the desired molecule to a solid support. This approach allows one to simply wash away unwanted contaminants leaving the essentially pure compound attached to the solid support. This technique can be advantageous when the desired product and the contaminants are difficult to separate using standard separation procedures such as extraction or chromatography. See WO 02/070020 and WO 99/18053 for additional discussion of the advantages relating to solid-phase synthesis.
In addition, organic synthesis on insoluble supports is a rapidly developing methodology which offers several advantages compared to traditional synthesis in solution. In recent years many new synthetic methods for solid-phase synthesis have been developed, and this technique is becoming a valuable alternative to traditional synthesis. Solid-phase synthesis is particularly useful when large numbers of different compounds in small quantities are needed for screening assays. Combinatorial chemistry and the production of compound libraries are usually based on solid-phase synthesis.
Therefore, the need exists for a procedure to prepare radiolabeled molecules and biosequences in high chemical purity and isotopic purity. Furthermore, there is a need for precursors to radiolabeled molecules that have a long shelf-life. The present invention fulfills the above-mentioned needs and has other related advantages.